The start of school this month will mark the return of a familiar sight: kids bent out of shape by the weight of their backpacks.

Experts have warned in recent years that kids are overloading their backpacks with heavy textbooks and personal items. Many believe that could result in an epidemic of back problems down the road. Others maintain that young skeletal systems won’t be adversely affected by loads.

As researchers continue to hash out the issue, many pediatricians are advising parents not to take any chances and to take steps to protect their kids just in case.

The issue of backpack safety rose to national prominence after the U.S. Consumer Product Safety Commission reported 12,688 backpack-related injuries from 1999 to 2000. But further research has failed to find a definitive link between heavy backpacks and back pain.

A study published in the medical journal Pediatrics reviewed a sample of the injuries recorded in the safety commission’s National Injury Information Clearinghouse for those years. The review found that 28 percent of injuries involving tripping over a backpack and another 13 percent involved being hit with a backpack. Only 6 percent of the injuries occurred to the back while wearing a backpack.

But the clearinghouse includes only injuries that require a trip to the emergency room. That could miss much of the chronic back pain that pediatricians fear could result long-term from carrying heavy backpacks.

Subsequent studies have found mixed results. A 2003 University of Michigan Health System study concluded that backpacks don’t cause the stress and strain on young back that’s been suggested.

”There is no good scientific evidence to support the claim that schoolbag load is a contributing factor to the development of low-back pain in growing children,” says Dr. Andrew Haig, medical director of the University of Michigan Spine Program and lead author of the study.

Haig surveyed children on their use of backpacks and their back pain, then weighed kids and their backpacks one morning as they arrived for school. Virtually all of the students carried backpacks every day. Third graders carried about 5.7 percent of their body weight, while middle schoolers averaged 11.4 percent.

While one-third of the students reported some back pain, the study found no relationship between backpack weight and back problems. Haig says children’s activity level and weight may have more to do with back pain.

But Haig acknowledged that the backpack loads in his study may have been lighter than usual because the weigh-in occurred near the end of the school year after students had already returned textbooks and when homework loads were lighter. And that could have skewed the results.

In 2004, researchers from the University of California-Riverside found 64 percent of surveyed students 11 to 15 years old reported back pain and that the main predictor of pain was the weight of their backpack. As the ratio of backpack weight to body weight increased, so did the reports of pain. But the increase was so gradual, the researchers were unable to specify a safe relative backpack weight.

Dr. David Siambanes, who directed the study, said the investigation was prompted by the increased number of young patients at the university’s spine center.

”Kids were complaining about pain, about their backpacks,” he said. ”Kids complain all the time and nobody pays much attention, but this seemed like too much of the same thing.”

The researchers also noted that many of the schools had removed lockers because of vandalism or safety concerns. That meant many students had to carry all of their textbooks throughout the day.

Some parents have opted to give their children roller bags. However, experts warn empty roller bags can weigh up to 80 percent more than an empty backpack, and that kids tend to carry more things in a roller bag than they would in a backpack. That could leave kids hefting even greater loads when they inevitably have to lift the bag up the stairs or onto a bus.

And teachers complain that roller bags end up blocking the aisles in classrooms and turn hallways into obstacle courses between classes.

Some schools have offered parents the opportunity to purchase a second set of textbooks to keep at home, so kids wouldn’t have to lug their books back and forth. But experts are concerned that might discriminate against families that can’t afford them.

That’s left parents with more concerns than solutions, and pediatricians like Central Oregon Pediatric Associates’ Dr. John Evered with more questions than answers.

”The literature seems to diverge on the issue. There are some articles suggesting there is no permanent connection between backpack weight and back pain. There are others that suggest that clearly there are changes in gait,” he says. ”Whatever the story, there are some simple things you can do to minimize it.”

Those include wearing the backpack with both straps and using a waist belt to help control the load. Backpacks should be properly fitted, ending above the waist, and should have padded straps to distribute the weight. Students should load the heaviest books closest to the back and pack weights should be limited to a maximum of 15 percent of body weight.

Dr. Michael Tobey, a Bend chiropractor, hasn’t seen many patients with back problem stemming from carrying too much weight.

”I’d be more concerned about their posture while carrying their backpacks,” he says. ”They tend to bend way over to counterbalance the weight. That would cause more problems than the actual physical weight.”

Tobey says 90 percent of his patients have developed problems due to bad posture, and that starts when they are kids. ”After 30, 40 years of being hunched over, we start feeling it,” he says.